Neurological diseases and disorders affect brain function. Many efforts have been made to develop curative or ameliorative therapies for these diseases and disorders; however, no comprehensive or universally curative therapy has been developed, even though there are numerous pharmacotherapeutic approaches that have been proven to be effective against various different diseases and disorders.
Huntington's disease (HD) is an inherited disease of the brain that affects the nervous system. It is caused by a defective gene that is passed from parent to child. The HD gene interferes with the manufacture of a particular protein known as ‘huntington’ which appears to be crucial for proper brain development. The classic signs of HD include emotional, cognitive and motor disturbances. Huntington's is characterized by jerky involuntary movements (chorea), but sometimes causes rigidity without abnormal movements, changes in using the limbs (apraxia), loss of control of bodily functions and dementia, including a progressive deterioration of memory, speed of thought, judgment, and lack of awareness of problems and planning. There is no known cure for Huntington's disease. Although there are a number of medications to help control symptoms associated with HD such as emotional and movement problems, there is no treatment to stop or reverse the course of the disease. Huntington's disease has been recognized as a disease with a general membrane abnormality. A significantly elevated level and activity (10 fold increase) of Na,K-ATPase has been observed in membranes of erythrocytes and basal ganglia of Huntington's patients compared to that of normal (Butterfield D A, Oeswein J Q, Prunty M E, Hisle K C, Markesbery W R). Increased sodium, potassium adenosine triphosphatase activity in erythrocyte membranes in Huntington's disease. Ann Neurology, 4:60-62, 1978) fibroblast membranes obtained from the skin of Huntington's disease patients (Schroeder F, Goetz I E, Roberts E, Membrane anomalies in Huntington's disease fibroblasts. J. Neurochem. 43: 526-539, 1984).
Alzheimer's disease is a form of dementia—a neurodegenerative disease that damages the brain's intellectual functions (memory, orientation, calculation, etc.), but usually preserves its motor functions. In Alzheimer's disease, the mind gradually deteriorates, causing memory loss, confusion, disorientation, impaired judgment and other problems that may affect a person's ability to perform normal daily activities. The type, severity, sequence and progression of mental changes vary greatly. There is no known cure for Alzheimer's disease and no known way to slow its progression. For some people in the early or middle stages of the disease, medication such as tacrine may alleviate some cognitive symptoms. Aricept (donepezil) and Exelon (rivastigmine) are reversible acetylcholinesterase inhibitors that are indicated for the treatment of mild to moderate dementia of the Alzheimer's type. These drugs (called cholinesterase inhibitors) work by increasing the brain's levels of the neurotransmitter acetylcholine, helping to restore communication between brain cells. Some medications may help control behavioral symptoms such as sleeplessness, agitation, wandering, anxiety, and depression. These treatments are aimed at making the patient more comfortable. Although no medication is known to cure Alzheimer's disease, cholinesterase inhibitors may improve performance of daily activities, or lessen behavioral problems. Medications for the treatment of Alzheimer's disease currently being tested include oestrogens, nonsteroidal anti-inflammatory agents, vitamin E, selegiline (Carbex, Eldepryl) and the botanical product gingko biloba.
Under normal conditions, neurons maintain their testing membrane potential and function regulated by membrane-bound, homeostatic, energy-dependent Na-K-ATPase pumps. Ischemia triggers alterations in ion homeostatsis potentially leading to irreversible tissue injuries. Compromised Na,K-ATPase activity has been suggested to play a role in a neuropathologic and apoptotic process in some models of focal ischemia and traumatic brain injury. The role of Na,K-ATPase in stroke-mediated ischemic brain injury has been reported to be associated with several different molecular mechanisms. Inhibition of Na,K-ATPase catalytic activity can, for example, lead to a reduction of ATP consumption during ischemia-reperfusion. Additionally, the deletion of cytosolic Ca2+ may cause neuronal cell death. Thus, inhibition of Na,K-ATPase, such as by cardiac glycosides, may result in an increase in intracellular Ca2+ levels and a decline in extrusion of intracellular Ca2+ via the Na—Ca exchanger. In line with this, the relatively lower levels of intracellular Ca2+ in hippocampal CA1 neurons has been observed three days after transient ischemia and elevation of calcium levels is believed to provide protection against delayed neuronal death across a wide range of post-ischemic treatment times.
One of the pharmacological mechanisms of action of cardiac glycosides involves their ability to bind to the ion exchange pump, Na, K-ATPase and to inhibit the activity of this particular enzyme. Na, K-ATPase, the transmembrane protein that catalyzes the active transport of Na+ and K+ across the plasma membrane, is a well established pharmacologic receptor for cardiac glycosides. This enzyme hydrolyzes ATP and uses the free energy to drive transport of K+ into the cell and Na+ out of cells, against their electrochemical gradients (Hauptman, P. J., Garg, R., and Kelly, R. A. Cardiac glycosides in the next millennium. Prog. Cardiovasc. Dis. 41: 247-254, 1999).
Na, K-ATPase is composed of two heterodimer subunits, the catalytic α-subunit and the glycosylated β-subunit. There is also a γ subunit, but it has not been studied in detail. The α-subunit has binding sites for ATP, Na+, K+, and cardiac glycosides. The β-subunit functions to stabilize the catalytic α-subunit and may play a regulatory role as well. Four different α isoforms (α1, α2, α3, α4) and three different β isoforms (β1, β2, and β3) have been identified in mammalian cells. The relative expression of each subunit type is markedly altered in normal and diseased states. Additionally, the apparent affinity of cardiac glycosides to the different α isoforms is quite different. Binding of cardiac glycosides to the α1 isoform is less than that which occurs with the α2 and α3 isoforms which are 250-fold or higher more sensitive to inhibition by this type of drug (Blanco, G. and Mercer, R. W. Isozymes of the Na, K-ATPase: heterogeneity in structure, diversity in function. Am. J. Physiol. 275 (Renal Physiol. 44): F633-F650, 1998). Sakai et al. (FEBS Letters 563: 151-154, 2004) report that expression of the α3 subunit isoform is increased in human colorectal cancer cells as compared to normal colorectal cells.
There is a broad range in relative water as opposed to lipid solubility of cardiac glycosides. While most cardiac glycosides can bind to and inhibit Na,K-ATPase activity, those cardiac glycosides which are relatively more water soluble (hydrophilic) than lipid soluble (lipophilic or hydrophobic) have only a limited ability to cross the lipid barrier to the brain known as the blood-brain bather. The blood-brain barrier (BBB) is a separation of circulating blood and cerebrospinal fluid (CSF) maintained by the choroid plexus in the central nervous system (CNS). Endothelial cells restrict the diffusion of microscopic objects (e.g. bacteria) and large or hydrophilic molecules into the CSF, while allowing the diffusion of small hydrophobic molecules (O2, hormones, lipid soluble cardiac glycosides, etc)
Nerium oleander is an ornamental plant widely distributed in subtropical Asia, the southwestern United States, and the Mediterranean. Its medical and toxicological properties have long been recognized. It has been used, for example, in the treatment of hemorrhoids, ulcers, leprosy, snake bites, and even in the induction of abortion. Oleandrin, an important component but not the sole component of oleander extract, is a cardiac glycoside.
Extraction of glycosides from plants of Nerium species has provided pharmacologically/therapeutically active ingredients from Nerium oleander. Among these are oleandrin, neriifolin, and other cardiac glycoside compounds. Oleandrin extracts obtained by hot-water extraction of Nerium oleander, sold under the trademark ANVIRZEL™, contain the concentrated form or powdered form of a hot-water extract of Nerium oleander. A Phase I trial of a hot water oleander extract (i.e. Anvirzel™) has been completed (Mekhail et al., Am. Soc. Clin. Oncol., vol. 20, p. 82b, 2001). It was concluded that oleander extracts, which would provide about 57 ug oleandrin/day, can be safely administered at doses up to 1.2 ml/m2/d. No dose limiting toxicities were found.
Huachansu is an extract obtained from toad skin and it comprises bufadienolides, such as bufalin, a cardiac glycoside. HuaChanSu is an approved medication for the treatment of cancer in China. It has been used to treat various cancers, including hepatic, gastric, lung, skin, and esophageal cancers.